The present invention relates to oxidation catalysts, and more particularly to oxidation catalysts for producing maleic anhydride from C.sub.4 to C.sub.10 hydrocarbons.
Essentially all of the catalysts and methods disclosed in the prior art for producing maleic anhydride from C.sub.4 and higher hydrocarbons employ oxidation catalysts containing vanadium in a valence state of less than +5. One method of forming such catalysts is to impregnate a catalyst base with a vanadium compound in which the vanadium has a valence of less than +5. Another more desirable method involves impregnating the catalyst with a vanadium compound which has vanadium in the +5 valence state and then reducing the vanadium from the +5 valence state to a valence less than +5.
Several references disclose oxidation catalysts containing vanadium-phosphorus mixed oxide catalysts and methods of preparing the same. For example, U.S. Pat. No. 4,179,404 discloses a process for preparing vanadium-phosphorus containing oxidation catalysts which consists of reducing pentavalent vanadium to a valence of less than +5 with a trivalent phosphorus compound. The phosphorus compound is employed in a concentration of from about 75 to 90 percent of the stoichiometric amount necessary to reduce the vanadium to a valency of from +5 to +4.
U.S. Pat. No. 4,153,577 discloses a catalyst complex useful for the partial oxidation of alkanes to the corresponding anhydrides in a vapor phase reaction. The oxidation catalyst used is a reduced vanadium and phosphorus mixed oxide catalyst containing either transition metals, Group IIA metals or rare earth metals.
Another oxidation catalyst suitable for preparing maleic anhydride from normal C.sub.4 hydrocarbons is disclosed in U.S. Pat. No. 4,123,388 which relates to a vanadium, phosphorus, copper mixed oxide complex containing an alkali or alkaline earth metal. In addition, tin is described as a desirable metal for incorporating into the catalyst.
U.S. Pat. No. 4,092,269 relates to vanadium-phosphorus oxidation catalysts wherein at least 20 atom percent of the vanadium is in the tetravalent state. A pore modification agent selected from polymeric materials, cellulosic materials, monosaccharides, etc. is added to the catalyst to provide pore diameters between 0.8 to 10 microns. The catalyst is described as useful for the conversion of aliphatic hydrocarbons to maleic anhydride.
U.S. Pat. No. 3,915,892 discloses a method of preparing a vanadium-phosphorus mixed oxide oxidation catalyst utilizing three bulk phase transitions, wherein the average valence of vanadium is maintained in the range of 4.1 to 4.5 and in addition a partial pressure of oxygen is maintained in contact with the mixed oxides formed.
As a rule, the prior art has avoided the use of crystalline aluminosilicate zeolites as support materials in catalysts for the production of maleic anhydride. U.S. Pat. Nos. 3,888,886 and 4,165,299, however, vaguely mention "zeolite" and "aluminosilicates," respectively, as possible choices among many catalytic carrier materials for the oxidation of butane to maleic anhydride. But these teachings offer nothing to suggest how a crystalline aluminosilicate zeolite can be employed without the adverse effects so often encountered with their use in the prior art. Indeed, it is not even certain if the teachings in the aforementioned patents specifically refer to crystalline aluminosilicate zeolites.
Accordingly, it is an object of the invention to provide zeolite catalysts, and methods for their preparation and use, which are useful for the oxidation of C.sub.4 to C.sub.10 hydrocarbons to maleic anhydride.
It is yet another object to provide catalysts for the production of maleic anhydride which have a relatively high surface area and comprise vanadium and phosphorus components supported on a carrier containing a crystalline aluminosilicate zeolite.
It is yet a further object of the invention to provide catalysts for producing maleic anhydride comprising vanadium and phosphorus and a microporous crystalline silica, such as silicalite.
It is yet a further object to provide a method for producing such catalysts, with the vanadium having an average valence in the range of +3.50 to +4.95.
These and other objects of the invention will become more apparent in view of the following specification and claims.